The candidate is an accomplished gastroenterology fellow with formal training in epidemiological methods and a commitment to a career in clinical and translational research in the study of inflammatory bowel disease (IBD). The candidate's long-term career goal is to become an independently funded physician investigator, devoting more than 75% of his time to establish and maintain a clinical research program in inflammatory bowel disease (IBD). The candidate's short-term career goals are to 1) build large analytic datasets based upon administrative data collected in registries; 2) design and interpret genetic data, including application of gene-environment interaction methodology; 3) oversee the collection of new bio specimens and understand the complexities of efficient bio banking; 4) acquire skills in the analysis and interpretation of high dimensional data sets, including genetic polymorphisms and gut micro biome sequence data; 5) produce the data and publications necessary for a successful R01 application. This application outlines the institutional commitment, research plan, career development activities and key mentors involved to ensure the candidate accomplishes these goals. In prior work, the candidate has demonstrated that use of oral contraceptives (OC) is associated with an increased risk of developing CD. This finding, in agreement with several other prior studies, has convincingly established OC as an etiologic risk factor for CD. Nonetheless, data regarding an influence of OC on CD progression and potential biological mechanisms underlying associations with CD are lacking. In this proposal, the candidate seeks to fill this knowledge gap through a comprehensive examination of the complex interaction between OC, sex steroid biomarkers, genetic risk loci, and alterations in the intestinal microbial environment in the etiology and progression of CD. The specific goals of this study are to 1) estimate the relative and population attributable risk associated with OC of developing CD or developing CD-related complications among patients with CD using comprehensive Swedish nationwide registries; 2) evaluate the interaction between OC and CD risk loci on incident CD and evaluate the relationship between prediagnostic plasma sex steroid hormones on risk of CD using two large, well-characterized cohorts which have provided detailed and updated lifestyle data for over 30 years; 3) evaluate the effect of OC use on gut microbial composition and mucosal inflammation within clinic-based patient cohorts. As an integral part of this proposal, the candidate's career development will be complemented by participation in advanced coursework and research seminars to develop expertise in genetic epidemiology, plasma biomarker, and micro biome data analysis. A formal mentorship committee and advisory team, consisting of multidisciplinary experts in epidemiology, genetics, genetic epidemiology, and the micro biome will provide supervision, guidance, and assistance for the candidate to achieve his goals. The research environment, which includes the MGH Crohn's and Colitis Center, the NIH-supported Center for the Study of IBD, the Channing Laboratory of chronic disease epidemiology, the Harvard School of Public Health, and the Karolinska Institute, will provide a rich, collaborative, and intellectually stimulating atmosphereto ensure the candidate's success. Through this award, the candidate will emerge as an independent clinical investigator by contributing to our understanding of the role of OC in the etiopathogenesis of CD through a series of studies that bridge epidemiologic correlation with disease causation.